The present invention relates to a thin film comprising titanium oxide as its main component with a high refractive index and low extinction coefficient and a sintered compact sputtering target comprising titanium oxide as its main component which is suitable for producing the foregoing thin film.
In recent years, technology of high density recording optical disks, which are high density optical information recording mediums capable of rewriting without using a magnetic head, has been developed, and these optical disks are being commercialized at a rapid rate. In particular, ever since its appearance in 1977 as a rewritable CD, CD-RW is the most popular phase-change optical disk. The rewrite cycle of a CD-RW is approximately 1000 times.
Moreover, DVD-RW for use as a DVD has been developed and commercialized, and the layer structure of this disk is basically the same as or similar to a CD-RW. The rewrite cycle of a DVD-RW is approximately 1000 to 10000 times.
These are electronic parts that record, reproduce and rewrite information by irradiating a laser beam and optically changing the transmittance, reflectance and the like of the recording material, and have spread rapidly.
Generally speaking, a phase-change optical disk used as a CD-RW, a DVD-RW or the like has a four-layer structure in which both sides of a recording thin film layer based on Ag—In—Sb—Te, Ge—Sb—Te or the like with a high melting point dielectric protective layer made of ZnS, SiO2 or the like, and which is additionally provided with a silver or silver alloy or aluminum alloy reflective film. Moreover, in order to increase the rewrite cycle, an interface layer is added between a memory layer and a protective layer as needed.
A reflective layer and a protective layer are demanded of optical functions of increasing the difference of reflectance between the amorphous part and crystal part of the recording layer, and additionally demanded of humidity resistance of the recording thin film and a function for preventing deformation caused by heat, and a function of controlling the thermal conditions upon recording (refer to Non-Patent Document 1).
Recently, a single-sided, dual layer optical recording medium has been proposed to realize larger capacity and higher density, (refer to Patent Document 1). In Patent Document 1, there is a first information layer formed on a substrate 1 and a second information layer formed on a substrate 2 from the incident direction of the laser beam, and these layers are affixed to each other with an intermediate layer interposed therebetween so that the information films face each other.
In the foregoing case, the first information layer is configured from a recording layer and a first metal reflective layer, and the second information layer is configured from a first protective layer, a second protective layer, a recording layer, and a second metal reflective layer. In addition, layers such as a hard coat layer and a thermal diffusion layer may be arbitrarily formed for protection against scratches, contamination and the like. Moreover, various materials have been proposed as the protective layer, recording layer, reflective layer and other layers.
A high melting point dielectric protective layer requires tolerance against repeated thermal stress by warming and cooling, requires the thermal effect not to affect the reflective film or other locations, needs to be thin with low reflectance, and requires strength that is free from deterioration. In this respect, the dielectric protective layer plays an important role. The recording layer, reflective layer, interference film layer and the like are similarly important in that they exhibit their functions respectively in the foregoing electronic parts such as CDs and DVDs.
The respective thin films with the foregoing multilayer structure are normally formed with the sputtering method. The principal of the sputtering method is as follows; specifically, the sputtering method causes a substrate and a target as a positive electrode and a negative electrode to face each other, and applies a high voltage between the substrate and the target under an inert gas atmosphere in order to generate an electric field. Here, the ionized electrons and inert gas collide and form plasma, the positive ions in the plasma collide with the target (negative electrode) surface and sputter the atoms configuring the target, and the sputtered atoms adhere to the opposing substrate surface so as to form a film.
In the foregoing circumstances, a target using titanium oxide (TiOx) has been proposed as a sputtering target for forming a heat ray reflective film and an antireflection film (refer to Patent Document 2). Here, the specific resistance value is set to 0.35 Ωcm or less to stabilize the discharge during sputtering. Consequently, DC sputtering is enabled and a film with a high refractive index can be obtained.
Here, the transmittance of the film will deteriorate, so the oxygen content is set to be 35 wt % or higher, and measures for introducing oxygen are additionally adopted. And, since the deposition rate will deteriorate due to the introduction of oxygen, a metal oxide is added to improve the deposition rate. However, there are problems in applying the target of Patent Document 2 to precision optical members and electronic parts that require films with a high refractive index and low absorption. Particularly on the short-wave length side in the vicinity of 400 nm is considered problematic.    [Patent Document 1] Japanese Published Unexamined Patent Application No. 2006-79710    [Patent Document 2] Japanese Patent No. 3836163    [Non-Patent Document 1] Technical Journal “Kogaku” Volume 26, Issue 1, Pages 9-15